1. Field of the Invention
The present invention relates to an optical waveguide device used in optical communications and the like, and to a method of adjusting a transmission loss of the optical waveguide device. To be more specific, the present invention relates to the optical waveguide device equipped with a plurality of optical waveguides such as arrayed waveguide gratings, optical splitters, and optical star couplers, and also relates to the method of adjusting the transmission loss of the optical waveguide device.
2. Description of the Related Art
In wavelength division multiplexing (WDM) transmission systems, arrayed waveguide gratings are generally used as either optical demultiplexers or optical multiplexers. It should be noted that an arrayed waveguide grating will be abbreviated as an “AWG” hereinafter. A basic structure of an AWG is described in, for example, a related art document 1, namely, Japanese Patent Application Laid-open No. 2000-221350 (refer to FIG. 10). FIG. 8 is a plan view for showing a basic structure of a general-purpose AWG. A description is made of the basic structure of the AWG based upon this drawing. The AWG 100 is constructed in such a manner that an input waveguide 101, “n” (symbol “n” is an integer equal to or larger than 2) output waveguides 102-1 to 102-n, a plurality of arrayed waveguides 103, a slab waveguide 104, and another slab waveguide 105 are formed on a substrate 106. The slab waveguide 104 connects the input waveguide 101 to the arrayed waveguides 103. The slab waveguide 105 connects the arrayed waveguides 103 to the output waveguides 102-1 to 102-n.
Next, operations of the AWG 100 will now be explained. The AWG 100 functions as an optical demultiplexer. First, a wavelength division multiplexing (WDM) signal beam “LO” (having wavelength of λ1, λ2, λ3, . . . , λn) is inputted via the input waveguide 101 to the AWG 100. Accordingly, the WDM signal beam LO is diffracted within the slab waveguide 104 to be widened, and then, the widened WDM signal beams LO are outputted to the respective arrayed waveguides 103. Lengths of adjoining waveguides of these arrayed waveguides 103 are different from each other. As a result, the respective WDM signal beams propagated through the arrayed waveguides 103 produce phase differences at the respective output terminals of the arrayed waveguides 103. As a consequence, the WDM signal beams outputted to the slab waveguide 105 may induce multiple beam interference in this slab waveguide 105. Then, signal beams having the same wavelengths are condensed to the respective input terminals of the output waveguides 102-1 to 102-n, and the condensed signal beams are outputted to the respective output waveguides 102-1 to 102-n. As a result, signal beams L1, L2, L3, . . . , Ln having wavelengths λ1, λ2, λ3, . . . , λn respectively, which are different from each other, are individually outputted from the respective output waveguides 102-1 to 102-n.
It should be noted that since the output waveguides 102-1 to 102-n are employed as waveguides for inputting and the input waveguide 101 is employed as an waveguide for outputting, the AWG 100 may be also operated as an optical multiplexer. Accordingly, the signal beams L1, L2, L3, . . . , Ln having the respective wavelengths are inputted to the output waveguides 102-1 to 102-n respectively, so the WDM signal beam “LO” is outputted from the input waveguide 101.
However, the AWG 100 has a wavelength dependence. To be specific, intensity of the signal beams L1 (λ1), L2 (λ2), L3 (λ3), . . . , Ln (λn) outputted from the AWG 100 is not equal to each other. FIG. 9 is a graph for representing transmission losses with respect to each of ports (i.e., respective wavelengths) that a general-purpose AWG has. It should be noted that a transmission loss is assumed as a loss of optical power, which is produced when a signal beam passes through an optical component. As previously explained, the transmission losses are different from each other for every wavelength to be outputted. This is because light propagated through a slab waveguide has an intensity distribution, so there is such a trend that the intensity of the light propagated closer to the vicinity of a center of the output waveguides becomes stronger, whereas the intensity of the light propagated closer to a peripheral portion of the output waveguides becomes weaker. As a consequence, there is a trend that the transmission loss of the wavelengths closer to the vicinity of the center of the output waveguides is smaller, whereas the transmission loss of the wavelengths closer to the edge thereof is larger. On the other hand, in a WDM transmission system, such a condition that intensity of signal beams having respective wavelengths is equal to each other is desirable in order to maintain a transmission quality. As a consequence, in order to compensate fluctuations in the transmission losses, optical attenuators and the like must be mounted on the respective ports of the AWG, and thus, transmission losses of the respective ports must be equalized with each other. This has been a cause of making the entire module bulky, requiring high cost.
Under such the circumstance, ideas for solving the above-explained problems have been proposed in the related art publication 1 (refer to FIG. 8). FIG. 10 is a plan view for showing an optical waveguide structure applied to an optical waveguide device disclosed in the related art publication 1. The optical waveguide structure 110 is provided on the output waveguides 102-1 to 102-n of the AWG 100. The optical waveguide structure 110 is constructed of an output waveguide 102-m through which a signal beam “Lm” passes, and a cross waveguide 112-m which crosses with the output waveguide 102-m. It should be noted that symbol “m” indicates any integer which satisfies 1≦m≦n. The cross waveguide 112-m crosses with the output waveguide 102-m at a cross portion 113-m. As a result, the cross waveguide 112-m gives an excessive loss to the signal beam Lm. The optical waveguide structure 110 controls the transmission losses by controlling a cross angle “a2” of the cross portion 113-m.
However, the optical waveguide structure 110 has the below-mentioned problem. That is, it is difficult to obtain a desirable excessive loss amount by merely adjusting the cross angle “a2.” This is because there are possibilities that a tolerance for adjusting the excessive loss amount becomes excessively severe, and such a dynamic range having a sufficient excessive loss amount cannot be achieved. As a result, the difference in the transmission losses among the output waveguides cannot be sufficiently reduced. As previously explained, the previous proposals have such a problem that it is difficult to control the difference in the transmission losses among the plurality of optical waveguides in high precision.